It depends on your speakers, but the BEST way is not the easiest way. The best way requires an active crossover and proper testing equipment and intimate knowledge of the loudspeaker's drivers capabilities. The easy way (assuming your speakers allow it) is to use a Y-cable to drive two amplifier channels from one preamp channel and power the upper and lower crossover sections of a loudspeaker system separately. Your results might or might not be meaningfully different from just using the system conventionally in the second case.
If your speakers have a set of binding posts on the back, then yes, you can biamp.
Using "only" four channels of the XPA-5 won't be a problem. The fifth channel will quietly "sit it out" in reserve until you change you configuration some day. The power supply will happily give what it's got to the four channels being used without holding anything back for the unused fifth channel.
There are arguments for passive vs active biamping that I won't get into. I found that dividing the number of speakers in my towers between two different amp channels helped my XPA-2 control the speakers noticeably better.
XMC-1, (2) XPA-2s passively vertically biamped, XPA-3, XPA-100. All Amps Gen. 1 Definitive Technology BP-30TL towers, BPVX/P surrounds and CLR 3000 center. dbx DB-SW15 subwoofer. Oppo BDP-93, HDMI HTPC Morrow & Analysis Plus audio cables Pangea and PS Audio power cables Onkyo CP-1046F Turn Table, Sherbourn Pre-1 balanced output preamp Tice Solo AV power conditioner, CMX-2 line conditioners Acoustimac and ATS acoustic panels
Thanks for the replies, i was thinking about future configurations for another 2ch system, keeping the XPA-5 and the Studio60's and using the extra channels in the XPA-5 to bi-amp the speakers. i think the amp bi amping the studio 60s would be a sweet setup.
Theater: Denon 3310ci, Emotiva XPA-5, UPA-2,PS3 Slim, Monster Power 1600,iTunes via Apple TV3, Paradigm Studio 60 v.4,CC-590 v.5, Emotiva ERD-1(side), Emotiva ERD-1(rear) PW-2200 Sub, JVC DILA-250HD Projector, Draper 92" Clarion Fixed screen, DIY Acoustic Panels
2ch- Audiolab M-DAC, Paradigm Ref Active 20's, 24" Target Ref Speaker Stands, Blue Jeans LC-1, iTunes via Apple TV3